1. Field of the Invention
The invention relates to an electrolytic capacitor. Particularly, the invention relates to an electrolytic solution for electrolytic capacitor having a low impedance as well as good low temperature stability and other characteristics.
2. Description of the Related Art
A capacitor is a common electrical part and is generally used, in various electric and electronic products, mainly for power supply circuits, noise filters for digital circuits or the like.
Various types of capacitors are currently used. This invention pertains to an aluminum electrolytic capacitor. A typical aluminum electrolytic capacitor is fabricated by etching a high purity aluminum foil to increase its surface area, anodizing a surface of the aluminum foil to provide a dielectric surface film, preparing a laminate made up of the anode foil, an aluminum cathode foil which is opposed to the dielectric surface film of the anode foil and has etched surfaces, and a separator interposed between the anode and cathode foils, winding the laminate to provide an element of a wound structure, impregnating the element with an electrolytic solution, inserting the impregnated element in a case (typically made of aluminum), and sealing the case with an elastic sealant. There are electrolytic capacitors having structures other than such a wound structure.
In such an electrolytic capacitor, the characteristics of an electrolytic solution are important factors for deciding performance of the electrolytic capacitor. Particularly, miniaturization of electrolytic capacitors leads to the use of anode and cathode foils having a higher surface area by etching, which results in a high resistivity in the capacitors. Consequently, an electrolytic solution used in such a capacitor is always required to have a low resistivity (specific resistance) and a high electrical conductivity.
A typical electrolytic solution for a conventional electrolytic capacitor comprises a solvent made up of ethylene glycol (EG) as a main solvent and up to about 10% by weight of additional water, and an electrolyte dissolved in the solvent, the electrolyte being a carboxylic acid such as adipic acid or benzoic acid, or an ammonium salt thereof. Such an electrolytic solution has a specific resistance of the order of 1.5 .OMEGA.-m (150 .OMEGA.-cm).
Capacitors have to have a low impedance (z). The impedance of a capacitor is decided by various factors. For example, a capacitor having a larger electrode area shows a lower impedance and, accordingly, a larger capacitor naturally attains a lower impedance. In addition, there is approach to attempting to attain a lower impedance by improving a separator. However, particularly in smaller capacitors, the specific resistance of the electrolytic solution is a predominant factor for impedance.
Although electrolytic solutions of an aprotic type having a lower specific resistance (gamma-butyrolactone and the like) have recently been developed (JP-A-62-145713, JP-A-62-145714, and JP-A-62-145715), capacitors using an aprotic-type electrolytic solution have an unsatisfactory impedance compared to solid electrolytic capacitors using an electronic conductor which is known as a low specific resistance electrolyte.
In addition, an aluminum electrolytic capacitor has poorer low temperature stability due to use of an electrolytic solution and, in fact, has a ratio of impedance at -40.degree. C. to impedance at 20.degree. C. (Z(-40.degree. C.)/Z(20.degree. C.)) of about 40 at 100 kHz, which is significant.
On the other hand, water used as part of a solvent in an electrolytic solution for an aluminum electrolytic capacitor is a chemically active substance for the materials of the anode and cathode foils and, in general, tends to reduce the life of the electrolytic capacitor by acting on the anode and cathode foils (hydration reactions).